There are many applications for instruments capable of measuring the total organic carbon content of water (TOC) over a wide range of TOC values. The typical approach is to oxidize the carbon in the sample to carbon dioxide, and measure the latter. For analysis of the TOC of samples of relatively high purity, the sample may be exposed to UV energy (i.e.  less than 254 nm), typically provided by a mercury lamp, possibly in the presence of a TiO2 or other catalyst. The conductivity of a static sample (e.g., a sample taken from a process stream of interest and analyzed separately) can be monitored over time during the UV exposure to determine when the reaction is complete. See commonly-assigned U.S. Pat. No. 4,626,413, and others. Such instruments are capable of extremely accurate measurements of the TOC of water samples. However, the oxidation takes considerable time, so that it would be desirable to provide a faster-responding instrument. Furthermore, the relationship of the conductivity of the water sample to its carbon dioxide content is linear only at low concentrations, leading to complexity in analysis of samples of higher CO2 content.
Current wide range TOC analytical processes use reagents and/or catalysts, such as sodium persulfate and phosphoric acid, to oxidize organic compounds in the sample to carbon dioxide, using either UV energy (i.e.  less than 254 nm), typically provided by a mercury lamp, or heat, typically at least 100xc2x0 C., to effect oxidation. Oxidation times are typically 5 to 15 minutes and reagents have to be replenished frequently. Commonly the resulting CO2 is diffused across a membrane into a sample of ultrapure water, and the conductivity of the latter measured to determine the CO2 content; this technique again becomes increasingly complicated at higher CO2 concentrations.
As mentioned, the oxidation of carbon to CO2 can be stimulated in several ways. An additional variation is in the treatment of the sample. The sample can be held static in an oxidation cell, or the carbon in a flowing stream can be oxidized as it flows through an oxidation cell. In the latter case, the conductivity is commonly measured at the entry into and exit from the cell, so as to provide a measure of the change in conductivity and thus of the amount of CO2 formed. However, this technique can provide an accurate measure of TOC only if the reaction is completed or is completed to a known degree while the sample is in the cell; neither can be reliably assured.
In a non-catalyzed oxidation process, the sample is typically contained in a platinum crucible and heated to a high temperature, such as 600xc2x0 C. to 900xc2x0 C. The CO2 generated is usually measured by means of a NDIR instrument.
As mentioned, the conductivity of a water sample is usually measured to determine the CO2 content and thus measured its TOC. Other conventional CO2 detection techniques employ non-dispersive infrared (NDIR) or Fourier-transform infrared (FTIR) techniques. In another prior art technique, exemplified in publications by Bondarowicz and by Roehl and Hoffman, plasma-stimulated emission is used to measure the CO2 content. The TOC is oxidized by conventional means, and a RF-generated inductively coupled plasma (ICP), typically providing plasma temperatures of  greater than 2000xc2x0 C., is used to heat the resulting carbon to a temperature sufficient to emit radiation; an emission spectrometer is then used to measure the carbon content, and this value is used to determine the TOC.
In a second known technique, exemplified in a paper by Emteryd et al., an ICP is used to simultaneously oxidize the TOC to CO2 and to heat it to a temperature suitable for emission spectrometry.
In both cases, the plasma generated is too hot to allow direct physical attachment of a conventional CO2 detector (e.g., one employing non-dispersive infrared (NDIR), Fourier-transform infrared (FTIR), or conductivity-based techniques), limiting the analytical technique to non-contact optical techniques, such as analysis of the spectral emission or mass spectrometry. This is because the previously described plasmas are xe2x80x9cequilibriumxe2x80x9d plasmas, that is, in which the energy of the electrons of the plasma is in equilibrium with the rotational and translational energy of the gas molecules. Accordingly, the plasma is physically extremely hot.
The present invention also employs plasma oxidation techniques in connection with the measurement of the TOC of a gaseous sample or a sample of water, but does so in a significantly different way than in these two prior art techniques.
More specifically, it would be desirable to employ plasma oxidation as the technique for converting TOC in the sample to CO2, so as to obtain an instrument capable of analyzing widely-varying TOC contents, yet allowing accurate analytical techniques such as NDIR, FTIR, or conductivity-based techniques to be used to measure the CO2 thus produced.
It is therefore an object of the invention to provide a wide-range TOC analytical instrument employing plasma oxidation to convert the carbon in a sample to CO2, but doing so at temperatures sufficiently low to allow preferred techniques to be employed for measuring the CO2 thus produced, and doing so in a short time, so as to increase the efficiency of the instrument.
Other objects of the invention will be apparent from the following.
According to the invention, a dielectric barrier discharge (DBD), also known as a silent discharge (SD) or atmospheric pressure glow discharge (APGD), is provided to oxidize TOC in a gaseous or aqueous sample to CO2. DBDs are characterized by the presence of one or more insulating dielectric layers in the current path between metal electrodes in addition to the discharge gap. As known to those of skill in the art, the microdischarges generated in a DBD can be described as a weakly ionized, non-equilibrium plasma; in this connection, xe2x80x9cnon-equilibrium plasmaxe2x80x9d means that the mean energy of the electrons within the plasma is not in equilibrium with the vibration, rotation or translation energy of the bulk gas molecules. Therefore, the majority of the gas molecules stay at ambient temperatures while the electron energy is relatively high, so that the electrons of the plasma are effective in cleaving molecular bonds and thus driving the oxidation of TOC.
The DBD employed according to the present invention is referred to herein simply as xe2x80x9cthe plasmaxe2x80x9d. Table 1 lists some characteristic microdischarge properties in air at atmospheric pressure for a suitable plasma having a discharge gap of 1 mm (taken from literature).
As noted, the relatively low plasma temperature (that is, as compared to the plasmas discussed in the art cited above) of less than 50xc2x0 C. allows confinement of the plasma in a glass cell; however, since the plasma is non-equilibrated, as noted the mean electron energy in this type of discharge is still sufficient to cleave chemical bonds. This allows the plasma to be used to drive the oxidation, while permitting measurement of the CO2 thus generated by NDIR, FTIR, or conductivity-based techniques, thereby satisfying an important aspect of the invention.
More specifically, due to their suitable energy, the electrons in the plasma are capable of generating highly reactive species. The nature of these species mainly depends on the type of gas that is filling the discharge gap. In the case of oxygen, some of these species are excited oxygen, atomic oxygen, ozone, peroxides, etc. In particular, in the presence of water (vapor) and oxygen, large quantities of hydroxyl radicals are formed in high amounts responsive to the plasma discharge. Hydroxyl radicals are very strongly oxidizing, and are considered to be the main oxidizing species not only in DBD induced oxidation reactions but also in the photocatalytic oxidation reactions employing UV irradiation of an aqueous sample, as discussed above. Accordingly, in addition to other factors, the available amount of hydroxyl radicals determines the rate of oxidation. The more hydroxyl radicals are generated, the faster the oxidation of a certain sample will proceed. In addition, a DBD also causes the emission of UV light which in turn drives the oxidation process.
According to the invention, the sample can be injected directly into the plasma chamber, and the chamber can be directly coupled to the CO2-detecting analytical instrument with no loss of sample. Thus, highly accurate detectors such as NDIR or FTIR instruments can be used to measure the CO2 content and thus determine the TOC of the sample.
More specifically, as known to those of skill in the art, FTIR provides analysis for numerous compounds, and is thus useful in the experimental stage, so as to ensure that complete oxidation is taking place; in an appropriate production environment (as discussed further below), NDIR, which is specific for a single infrared absorbing substance (here CO2), may be preferred. Under certain circumstances, conductivity-based techniques may be preferred.
Still more specifically, the present invention provides for gas-phase plasma oxidation as well as for plasma oxidation of TOC in a gaseous or aqueous sample to CO2. Creation of the plasma requires only a supply of oxygen-containing gas, such as air, CO2-free air, synthetic air, an inert gas (i.e. helium, argon, xenon etc.) charged with oxygen, or pure oxygen, and application of high voltage electrical power. If the oxidation takes place in the gas phase in the energetic plasma, oxidation times are less than one minute. If the oxidation takes place in the liquid phase, oxidation times are usually longer, but are still typically less than two minutes, thus fulfilling the object of the invention for a fast-responding, wide range instrument. The explanation for the difference in oxidation times is that the active species, which are generated mainly in the gas phase or on the liquid surface of the sample, must diffuse into the liquid phase before the oxidation can take place. To hasten this process, a large surface area is favorable and therefore a large wettable glass surface is provided. The experiments of the present inventors, as summarized below, indicate that plasma-induced oxidation according to the invention is considerably faster than and applicable to a wider range of TOC contents than existing methods.
Detection can be accomplished with any conventional CO2 detection system, such as non-dispersive infrared (NDIR) or Fourier-transform infrared (FTIR) spectroscopy, or by dissolving the CO2-containing gaseous product in a sample of ultrapure water, and measuring the change in conductivity of the water sample; the CO2 content of the sample can be determined from the change in conductivity. Alternatively, the emission spectra of CO2 resulting from the organic carbon oxidation can be measured directly; the intensity of the emission is a function of the concentration of CO2 present.